Module polkadot_sdk_docs::guides::async_backing_guide

How to enable Async Backing on parachain projects that started in 2023 or before.

Upgrade Parachain for Asynchronous Backing Compatibility

This guide is relevant for cumulus based parachain projects started in 2023 or before, whose backing process is synchronous where parablocks can only be built on the latest Relay Chain block. Async Backing allows collators to build parablocks on older Relay Chain blocks and create pipelines of multiple pending parablocks. This parallel block generation increases efficiency and throughput. For more information on Async backing and its terminology, refer to the document on the Polkadot Wiki.

If starting a new parachain project, please use an async backing compatible template such as the parachain template. The rollout process for Async Backing has three phases. Phases 1 and 2 below put new infrastructure in place. Then we can simply turn on async backing in phase 3.

Prerequisite

The relay chain needs to have async backing enabled so double-check that the relay-chain configuration contains the following three parameters (especially when testing locally e.g. with zombienet):

"async_backing_params": {
    "max_candidate_depth": 3,
    "allowed_ancestry_len": 2
},
"scheduling_lookahead": 2

scheduling_lookahead must be set to 2, otherwise parachain block times will degrade to worse than with sync backing!

Phase 1 - Update Parachain Runtime

This phase involves configuring your parachain’s runtime /runtime/src/lib.rs to make use of async backing system.

1. Establish and ensure constants for capacity and velocity are both set to 1 in the runtime.
2. Establish and ensure the constant relay chain slot duration measured in milliseconds equal to 6000 in the runtime.

// Maximum number of blocks simultaneously accepted by the Runtime, not yet included into the
// relay chain.
pub const UNINCLUDED_SEGMENT_CAPACITY: u32 = 1;
// How many parachain blocks are processed by the relay chain per parent. Limits the number of
// blocks authored per slot.
pub const BLOCK_PROCESSING_VELOCITY: u32 = 1;
// Relay chain slot duration, in milliseconds.
pub const RELAY_CHAIN_SLOT_DURATION_MILLIS: u32 = 6000;

3. Establish constants MILLISECS_PER_BLOCK and SLOT_DURATION if not already present in the runtime.

// `SLOT_DURATION` is picked up by `pallet_timestamp` which is in turn picked
// up by `pallet_aura` to implement `fn slot_duration()`.
//
// Change this to adjust the block time.
pub const MILLISECS_PER_BLOCK: u64 = 12000;
pub const SLOT_DURATION: u64 = MILLISECS_PER_BLOCK;

4. Configure cumulus_pallet_parachain_system in the runtime.

• Define a FixedVelocityConsensusHook using our capacity, velocity, and relay slot duration constants. Use this to set the parachain system ConsensusHook property.

type ConsensusHook = cumulus_pallet_aura_ext::FixedVelocityConsensusHook<
	Runtime,
	RELAY_CHAIN_SLOT_DURATION_MILLIS,
	BLOCK_PROCESSING_VELOCITY,
	UNINCLUDED_SEGMENT_CAPACITY,
>;

impl cumulus_pallet_parachain_system::Config for Runtime {
    ..
    type ConsensusHook = ConsensusHook;
    ..
}

• Set the parachain system property CheckAssociatedRelayNumber to RelayNumberMonotonicallyIncreases

impl cumulus_pallet_parachain_system::Config for Runtime {
	..
	type CheckAssociatedRelayNumber = RelayNumberMonotonicallyIncreases;
	..
}

5. Configure pallet_aura in the runtime.

• Set AllowMultipleBlocksPerSlot to false (don’t worry, we will set it to true when we activate async backing in phase 3).

• Define pallet_aura::SlotDuration using our constant SLOT_DURATION

impl pallet_aura::Config for Runtime {
	..
	type AllowMultipleBlocksPerSlot = ConstBool<false>;
	#[cfg(feature = "experimental")]
	type SlotDuration = ConstU64<SLOT_DURATION>;
	..
}

6. Update sp_consensus_aura::AuraApi::slot_duration in sp_api::impl_runtime_apis to match the constant SLOT_DURATION

fn impl_slot_duration() -> sp_consensus_aura::SlotDuration {
	sp_consensus_aura::SlotDuration::from_millis(SLOT_DURATION)
}

7. Implement the AuraUnincludedSegmentApi, which allows the collator client to query its runtime to determine whether it should author a block.

   • Add the dependency cumulus-primitives-aura to the runtime/Cargo.toml file for your runtime

..
cumulus-primitives-aura = { path = "../../../../primitives/aura", default-features = false }
..

• In the same file, add "cumulus-primitives-aura/std", to the std feature.

• Inside the impl_runtime_apis! block for your runtime, implement the cumulus_primitives_aura::AuraUnincludedSegmentApi as shown below.

fn impl_can_build_upon(
	included_hash: <Block as BlockT>::Hash,
	slot: cumulus_primitives_aura::Slot,
) -> bool {
	ConsensusHook::can_build_upon(included_hash, slot)
}

Note: With a capacity of 1 we have an effective velocity of ½ even when velocity is configured to some larger value. This is because capacity will be filled after a single block is produced and will only be freed up after that block is included on the relay chain, which takes 2 relay blocks to accomplish. Thus with capacity 1 and velocity 1 we get the customary 12 second parachain block time.

8. If your runtime/src/lib.rs provides a CheckInherents type to register_validate_block, remove it. FixedVelocityConsensusHook makes it unnecessary. The following example shows how register_validate_block should look after removing CheckInherents.

cumulus_pallet_parachain_system::register_validate_block! {
	Runtime = Runtime,
	BlockExecutor = cumulus_pallet_aura_ext::BlockExecutor::<Runtime, Executive>,
}

Phase 2 - Update Parachain Nodes

This phase consists of plugging in the new lookahead collator node.

1. Import cumulus_primitives_core::ValidationCode to node/src/service.rs.

use cumulus_primitives_core::{
	relay_chain::{CollatorPair, ValidationCode},
	ParaId,
};

2. In node/src/service.rs, modify sc_service::spawn_tasks to use a clone of Backend rather than the original

sc_service::spawn_tasks(sc_service::SpawnTasksParams {
    ..
    backend: backend.clone(),
    ..
})?;

3. Add backend as a parameter to start_consensus() in node/src/service.rs

fn start_consensus(
    ..
    backend: Arc<ParachainBackend>,
    ..

if validator {
  start_consensus(
    ..
    backend.clone(),
    ..
   )?;
}

4. In node/src/service.rs import the lookahead collator rather than the basic collator

use cumulus_client_consensus_aura::collators::lookahead::{self as aura, Params as AuraParams};

5. In start_consensus() replace the BasicAuraParams struct with AuraParams
   • Change the struct type from BasicAuraParams to AuraParams
   • In the para_client field, pass in a cloned para client rather than the original
   • Add a para_backend parameter after para_client, passing in our para backend
   • Provide a code_hash_provider closure like that shown below
   • Increase authoring_duration from 500 milliseconds to 2000

let params = AuraParams {
    ..
    para_client: client.clone(),
    para_backend: backend.clone(),
    ..
    code_hash_provider: move |block_hash| {
        client.code_at(block_hash).ok().map(|c| ValidationCode::from(c).hash())
    },
    ..
    authoring_duration: Duration::from_millis(2000),
    ..
};

Note: Set authoring_duration to whatever you want, taking your own hardware into account. But if the backer who should be slower than you due to reading from disk, times out at two seconds your candidates will be rejected.

6. In start_consensus() replace basic_aura::run with aura::run

let fut =
aura::run::<Block, sp_consensus_aura::sr25519::AuthorityPair, _, _, _, _, _, _, _, _, _>(
   params,
);
task_manager.spawn_essential_handle().spawn("aura", None, fut);

Phase 3 - Activate Async Backing

This phase consists of changes to your parachain’s runtime that activate async backing feature.

1. Configure pallet_aura, setting AllowMultipleBlocksPerSlot to true in runtime/src/lib.rs.

impl pallet_aura::Config for Runtime {
	type AuthorityId = AuraId;
	type DisabledValidators = ();
	type MaxAuthorities = ConstU32<100_000>;
	type AllowMultipleBlocksPerSlot = ConstBool<true>;
	type SlotDuration = ConstU64<SLOT_DURATION>;
}

2. Increase the maximum UNINCLUDED_SEGMENT_CAPACITY in runtime/src/lib.rs.

mod async_backing_params {
	/// Maximum number of blocks simultaneously accepted by the Runtime, not yet included
	/// into the relay chain.
	pub(crate) const UNINCLUDED_SEGMENT_CAPACITY: u32 = 3;
	/// How many parachain blocks are processed by the relay chain per parent. Limits the
	/// number of blocks authored per slot.
	pub(crate) const BLOCK_PROCESSING_VELOCITY: u32 = 1;
	/// Relay chain slot duration, in milliseconds.
	pub(crate) const RELAY_CHAIN_SLOT_DURATION_MILLIS: u32 = 6000;
}

3. Decrease MILLISECS_PER_BLOCK to 6000.

• Note: For a parachain which measures time in terms of its own block number rather than by relay block number it may be preferable to increase velocity. Changing block time may cause complications, requiring additional changes. See the section “Timing by Block Number”.

mod block_times {
	/// This determines the average expected block time that we are targeting. Blocks will be
	/// produced at a minimum duration defined by `SLOT_DURATION`. `SLOT_DURATION` is picked up by
	/// `pallet_timestamp` which is in turn picked up by `pallet_aura` to implement `fn
	/// slot_duration()`.
	///
	/// Change this to adjust the block time.
	pub const MILLI_SECS_PER_BLOCK: u64 = 6000;

	// NOTE: Currently it is not possible to change the slot duration after the chain has started.
	// Attempting to do so will brick block production.
	pub const SLOT_DURATION: u64 = MILLI_SECS_PER_BLOCK;
}

4. Update MAXIMUM_BLOCK_WEIGHT to reflect the increased time available for block production.

const MAXIMUM_BLOCK_WEIGHT: Weight = Weight::from_parts(
	WEIGHT_REF_TIME_PER_SECOND.saturating_mul(2),
	cumulus_primitives_core::relay_chain::MAX_POV_SIZE as u64,
);

5. Add a feature flagged alternative for MinimumPeriod in pallet_timestamp. The type should be ConstU64<0> with the feature flag experimental, and ConstU64<{SLOT_DURATION / 2}> without.

impl pallet_timestamp::Config for Runtime {
    ..
    #[cfg(feature = "experimental")]
    type MinimumPeriod = ConstU64<0>;
    #[cfg(not(feature = "experimental"))]
    type MinimumPeriod = ConstU64<{ SLOT_DURATION / 2 }>;
    ..
}

Timing by Block Number

With asynchronous backing it will be possible for parachains to opt for a block time of 6 seconds rather than 12 seconds. But modifying block duration isn’t so simple for a parachain which was measuring time in terms of its own block number. It could result in expected and actual time not matching up, stalling the parachain.

One strategy to deal with this issue is to instead rely on relay chain block numbers for timing. Relay block number is kept track of by each parachain in pallet-parachain-system with the storage value LastRelayChainBlockNumber. This value can be obtained and used wherever timing based on block number is needed.